Quite frankly, you are a complete idiot to use a wireless pet fence system to contain a Boxer/Pitbull mix...and I say this having owned BOTH breeds.
Both breeds have an unfair reputation of being vicious. Many communities do not allow Pits and even one incidence of aggression is enough to require euthanasia coupled with a significant fine/jail time to the owner.
So...a UPS driver going up the driveway turns around and sees a large Boxer/Pit mix crouched down, looking to leap up to them...and you claim the driver is the aggressor because they used the only object they have available in an attempt to protect themselves.
Have some common sense! If you wish to keep this breed, build yourself a privacy fence and be save yourself the grief of having your socks sued off of you.